Here you go, stop complaining, and
check if you fulfill the requirements
Actually I think that by checking the ID you have more security

Anyhow, I agree that it might be annoying. I find myself annoyed since I live in Sweden, and everything I need is the card and my PIN number. But this is also a matter of national policy, and I think that regardless of the new legislation (which says that for debit cards the PIN number will be preferred over the signatura, while for credit cards, the only valid thing is the signature), the merchant still has the right to ask for the ID because if it there is a problem... that merchant will have to pay the loss. But again, little by little, I think that when you pay with debit cards, more merchants avoid asking for the ID (specially in big stores)
And what they do it is totally correct because, afaik it is not a valid id medium. If I remember correctly, if the ID is not mandatory and/or valid for travalling, then it is not valid. If you want to know which ones are likely to be accepted, have a look at
PRADO.
As an example, I use my DNI in Sweden every time I am asked for an ID, and I have no trouble.
There are two forms of invoices in Spain, one without any detail of the buyer, and another with all details, you got the latter one (of course if you get issued the former one, you can always ask for the latter). The latter one is usually issued when you get a service (repair) or some purchase of something 'big' (car, fridge... house

. They don't really need to prove that they sold it to you, it is actually for your own good (they will give the VAT you are paying anyways to the tax office regardless of the invoicing mode); but in any case, those details don't usually matter (unless it is a house or something like that).
Next time... tell them, I don't they will care much (but let's say that in the future you lose your invoice, and they would be the only ones to give you a copy, that, without those details they wouldn't be able to... so it's up to you).
I think there is an idea of an EU-ID, but I doubt it will ever happen, too many countries are against that (look at UK... you had it, and then you got rid off it).
About the latter comment, I won't say much because it denotes (in my humble opinion) some xenophobia.
Check the link about the requirements for the corresponding card
Like every other country in the EU, including UK. All countries pay and get back; some get back more than what they pay (but I think Spain is past that, now it is paying more than what it gets back).
There was one... but thanks to the (mistakenly) territorial system, Spain has these regions... and the 'competences' (I don't think this is the right word... but I cannot think of anything better right now) were transferred to the regional governments. But besides that, there is a national framework taking care that the quality is the same among all regions.
But again, each country decides this. Sweden has regional health systems too, and it is not any better or worst than Spanish one. It is the way it is.
I agree and finally, with the new decrees from the government there will be one unique medical record, for once and for all (and that includes one medical card).
Each law enforcement is in charge of something different. You might think they do the same things, but they do not. And I presume they have their own databases, but these are seamlessly connected (when it is about security and money, both the polices and the tax, social security, etc have a great collaboration...)
Well... the 'Policía' is a civil law enforcement, while the 'Guardia Civil' is a militar one... that might be tricky although it is aimed in the future.
Just some notes, and I'm done because this is not the place to do this. This is a forum to talk about mac... rumors!
And yes... I'm excited about the effort of Apple in Spain, which means that Apple believes in Spain in the medium-long term; which also means that I will be able to eventually go back to Spain and live there

with my mac bought right at the closest Apple Store (what about the rumours of the Apple Store in Valladolid... at the mall opened recently with IKEA?)